This invention relates to the preparation, storage and transportation of waxy hydrocarbon mixtures. In this field, a hydrocarbon mixture is distilled to produce an overheads fraction which is used as the carrier fluid for the heavier fractions of the distillation process. The heavier fractions are carried in the overhead in the form of congealed particles having diameters of about 0.05 mm to about 20 mm or more. These particles are formed by such processes as prilling, extruding or beading and the resulting slurry is pumped in a transporting system that includes a pipeline. Upon pumping, these congealed particles partially dissolve with time and mixing to leave isolated wax crystals in the carrier fluid.
In order to maximize the percent of each hydrocarbon mixture that can be processed into a transportable slurry, as much overhead as economically feasible must be distilled. Prior art approaches to obtaining economically pumpable slurries have included methods designed to determine the maximum amount of overhead that can be obtained from the hydrocarbon without creating a slurry which has too large a pressure drop when pumped. Past methods include the cloud point or pour point to determine the distillation cutpoint for the overhead. In the pour point methods, the cutpoint is generally at a fraction whose pour point is about 5.degree. F. to about 10.degree. F. below the average temperature of the transporting system.
Experimental data has shown that the use of the cloud point or pour point to determine the distillation cutpoint is unreliable. In some cases, a low viscosity overhead with a pour point higher than the slurry pumping temperature contributed little to the slurry pressure drop whereas a lower pour point overhead (10.degree. F. blow the slurry pumping temperature) contributed significantly to the pressure drop within the pipeline. In particular, methods that use the cloud point or pour point to determine the maximum amount of overhead that can be taken do not consider the effect of wax crystal type on slurry pumpability. Wax crystals formed in the overhead can be substantially different than those formed in the congealed particles of the heavy fractions, and the presence of wax crystals from the overhead greatly increases the slurry pressure drop. After the congealed particles partially disintegrate leaving isolated wax crystals, some wax crystals from the overhead are still detrimental to the slurry pumpability.
Processes such as flash distillation can have a large overlap in the cuts and can result in overheads with a great number of wax crystals. The wax crystals are from fractions that boil within the range of the heavy cuts and crystallize in the overhead at the slurry transporting temperature. Additives do not affect the amount of wax crystals in the overhead but can modify the crystal structure of the wax which crystallizes in the presence of these additives and be beneficial in some, but not all cases. Regardless of the cause, these wax crystals are more economically conveyed in the congealed particles of the heavy fractions than in the overhead and should be minimized in the overhead.
U.S. Pat. No. 4,149,756 teaches a method of optimizing the pumpability of a hydrocarbon slurry by determining the optimum distillation cutpoint of a hydrocarbon mixture as the distillation temperature at which there is a substantial increase in overhead viscosities and/or pressure drop during pipelining of slurries comprising the overhead. The molecular carbon number distribution of the overhead at the optimum distillation cutpoint is determined and compared with the molecular carbon number distribution of the overhead in the distillation process. The distillation process is then adjusted to obtain an overhead fraction having substantially the same molecular carbon number distribution as the overhead taken at the optimum distillation cutpoint.
The present invention may be used in connection with that of U.S. Pat. No. 4,149,756 or may be used independently of that invention. In the method of the present invention, the pumpability efficiency of a hydrocarbon slurry is determined prior to stabilization of the slurry and prior to transporting the slurry through a pipeline by measuring the wax crystal content of the slurry, and then utilizing the known wax crystal content to determine the expected pressure drop that the slurry will experience when transported through a particular pipeline. The wax crystal content of the slurry may then be modified or controlled to obtain desirable or optimum slurry pumpability conditions for the pipeline. The present invention permits the design of a variety of slurry systems, each employing differing waxy hydrocarbon mixtures having a variety of wax crystal characteristics in a particular pipeline prior to actually pumping the slurries in the pipeline and encountering difficulties with slurry pumpability.